Keto acids

Pyruvic acid (α-keto carboxylic acid, top), acetoacetic acid (β-keto carboxylic acid, middle) and levulinic acid (γ-keto carboxylic acid, bottom).

Keto acids , also called keto carboxylic acids , are carboxylic acids that contain an additional keto group . They belong to the oxocarboxylic acids . Their properties are determined by the distance between the two functional groups . The distance is often denoted by Greek letters ( α-constant = 1,2-distance, β-constant = 1,3-distance etc.).

biochemistry

In cellular processes, keto acids play a central role in amino acid metabolism and in maintaining the redox status . α-Keto acids contain the carbon structure of the analogous α-amino acid. β-Keto acids are energy-rich (unstable) metabolites that easily enter into reactions with a loss of CO ₂ ( decarboxylation ).

Typical biochemical reactions of the α- and β-keto acids. Metabolites (blue) from left to right: Lac, lactate; Ala, alanine; Pyr (pyruvate, a prototype α-keto acid); PEP, phosphoenolpyruvate; OA, oxaloacetate (α / β-keto acid; instability due to β-keto function); Mal, malate; AcCoA, Acetyl Coenzyme A. Enzymes (green) from left to right: LDH, lactate dehydrogenase; PC, pyruvate carboxylase; MDH, malate dehydrogenase (part of the malate-aspartate shuttle system); PGT, glutamate / pyruvate transaminase; PK, pyruvate kinase (gain of 1 ATP); PPD, pyruvate / phosphate dikinase ( C4 plants ; use of two high-energy bonds from ATP and an inorganic phosphate); PEP-CK, phosphoenolpyruvate carboxykinase (conversion of OA to PEP); PDH, pyruvate dehydrogenase multienzyme complex (prototype of an oxidative decarboxylation reaction), ME, malate enzyme (when coupled with MDH, NADH, H ⁺ → NADPH, H ⁺ conversion is possible).

α-keto acids

Pyruvic acid with its salts, the pyruvates, is the simplest α-keto acid. Under the action of a pyruvate decarboxylase , pyruvic acid can be decarboxylated to acetaldehyde (and CO ₂ ) during alcoholic fermentation or to acetyl-CoA (and CO ₂ ) in the multi-enzyme complex of pyruvate dehydrogenase . In the citric acid cycle there is another reaction based on the same basic principle: the decarboxylation of the α-ketoglutarate to succinyl-CoA . Coenzyme this as " oxidative decarboxylation " processes are called thiamine pyrophosphate and NAD ⁺ , possibly coenzyme A . Under anaerobic conditions, pyruvate is reduced to lactate in the mammalian organism (e.g. in muscles during intensive use).

Also, in vitro an analogous reaction can be observed: The relatively weak C-C bond in the moiety R-CO-COOH can be prepared by addition of concentrated sulfuric acid are cleaved and there are carbon monoxide and the corresponding carboxylic acid R-COOH.

Another typical reaction of α-keto acids is transamination , which involves a reciprocal amination with simultaneous deamination of glutamic acid without the occurrence of free ammonia. Pyruvate becomes alanine , oxaloacetate (a compound that is both α- and β-keto acid) becomes aspartate, and α-ketoglutarate becomes glutamate , while the coenzyme is pyridoxal phosphate .

β-keto acids

The simplest β-keto acid is the volatile acetoacetic acid . β-Keto acids are mostly unstable substances that break down with decarboxylation. This process can take place spontaneously, also catalyzed biochemically in the cell. One example is the decarboxylation of oxaloacetate in gluconeogenesis by phosphoenolpyruvate carboxykinase (PEP-CK) or by the malate enzyme (ME). Acetoacetic acid breaks down into acetone and carbon dioxide .

γ-keto acids

The simplest γ-keto acid is levulinic acid .

Individual evidence

1. ^ Albert Gossauer: Structure and reactivity of biomolecules , Verlag Helvetica Chimica Acta, Zurich, 2006, pp. 362–363, ISBN 978-3-906390-29-1 .
2. ^ ^{A b} Albert Gossauer: Structure and reactivity of biomolecules , Verlag Helvetica Chimica Acta, Zurich, 2006, p. 362, ISBN 978-3-906390-29-1 .
3. ^ Brockhaus ABC Chemie , VEB FA Brockhaus Verlag Leipzig 1965, p. 670.